Filtration device with pressure-activated means for bypassing serial filter layers

ABSTRACT

A serial-flow filtration device is provided with pressure-activated means for by-passing at least one of its constituent layer (or layers) of filtration material. The filtration device comprises a housing, a fluid inlet, a fluid outlet, and at least an upper and a rearmost filter layer. The upper filter layer divides the interior of the housing into an upstream zone and a downstream zone, with the rearmost filter layer residing in the downstream zone. The fluid inlet enables the introduction of fluid into said upstream zone. The fluid outlet enables releases of the fluid from said downstream zone. The upper filter layer, but not the rearmost filter layer, has integrated thereinto a pressure-activated means capable of allowing substantially non-selective passage of fluid from the upstream zone into the downstream zone upon the attainment of a predetermined pressure differential across said upper filter layer. The bypass means can be embodied, for example, as a pressure-breachable seal or as a pressure-activated gate.

FIELD

[0001] In general, the present invention is directed to filtrationdevices comprising serially-arranged filter layers, and moreparticularly, wherein the filtration device further comprises bypassmeans responsive to undesirably high pressure differentials across atleast one of said filter layers.

BACKGROUND

[0002] Filtration devices comprising a plurality of discrete filterlayers are well-known. Several varieties exist.

[0003] The filtration devices of interest to the present invention arethose wherein several filter layers are arranged in a stack or bankwithin a common housing, such that fluid brought into the housing (i.e.,through an inlet) passes through each filter layer sequentially prior tobeing released from said housing (i.e., through an outlet). Such“serial-flow” filtration devices may include other components,independent of the filter layers, that assist or have an influence onthe flow path of fluid within the housing.

[0004] According to a common configuration, the filter layers in thefiltration device are stacked such that the retentiveness of theconstituent layers define a gradient from low to high. In such gradientfilter devices, the foremost filter layer—i.e., the layer first impingedupon by fluid introduced into the housing—typically has the lowestretentiveness, whereas the subsequent layers are sequentially moreretentive. In another common configuration, the retentiveness of eachconstituent filter layer is essentially the same, the layers typicallybeing arranged with an eye towards the efficient use of a tight andlimited space.

[0005] Serial-flow filtration devices are employed for variousapplications. Types of industrial applications include, for example,pharmaceutical manufacture, processing blood plasma or serumfractionation products, ophthalmic solution manufacture, the manufactureof specialty chemicals, and the like. In industrial applications, thefiltration devices are typically configured for so-called “primary orsecondary clarification”, i.e., the initial filtration of a fluid priorto further downstream cleaning and polishing processes. In suchapplications, the fluid is often handled in large batches (e.g., in theorder of several thousands of liters) and typically has high solidcontent.

[0006] A key concern in the conduct of serial filtration is pressuremanagement. As fluid enters the device, solids will generally beretained and accumulated more so on the foremost filter layer. Uncheckedsedimentation on a filter layer will eventually give rise to so-called“cake” formation. This and like formations effectively decrease theporosity of the filter medium (cf., clogging), such that—given constantflow of fluid into the housing—upstream pressure will rise. If pressurereaches a certain level, the filtration process will either have to beterminated, for example, to replace, clean, or revitalize the “spent”filter layer. Otherwise, one risks catastrophic filter component failureand/or otherwise compromises or ruins one's filtration product orresult.

[0007] When the filtration process is terminated, labor, time, andmaterial resources (e.g., replacement components, cleaning fluids, etc.)need to be expended. Of particular note, in respect of labor, is therather onerous task of disassembling and reassembling anindustrial-sized filtration device. Stacks of large filter componentswhen soaked with fluid are quite heavy, unwieldy, and often messy.Reducing the frequency with which such maintenance has long been andcontinues to be highly desirable.

[0008] In light of the above, for filtration devices employingserially-arranged filter layers, a need exists for automatically passinga clogged or otherwise spent filter layer at a prescribed pressuredifferential thereacross in a manner that is reasonably reliable and notinordinately expensive to implement.

SUMMARY

[0009] In response to the above need, the present invention provides afiltration device having pressure-activated means for by-passing aclogged, dirty, or otherwise spent filter layer, and by doing so,reducing the pressure in said device upstream of said filter layer,thereby extending the operative life of the filtration device.

[0010] The filtration device comprises a housing, a fluid inlet, a fluidoutlet, and at least an upper and a rearmost filter layer, the filterlayers being made of the same or different selectively-permeablefiltration material. The upper filter layer divides the interior of thehousing into an upstream zone and a downstream zone. The fluid inletenables the introduction of fluid into said upstream zone. The fluidoutlet enables the release of fluid from said downstream zone. Therearmost filter layer is positioned in the downstream zone and willtypically, but not always, bears some similarity to the upper filterlayer in respect of porosity, basic functionality, construction, and thelike. In accord with the invention, however, the upper filter layer—butnot the rearmost filter layer—has integrated thereintopressure-activated bypass means.

[0011] The bypass means has two principal embodiments. In the first, themeans are provided by constructing and assembling a filter layer intothe device housing such that it forms a pressure-breachable sealtherewith. In the second, the means are created by the integration intothe filter layer of a pressure-activated gate. In both embodiments, thebypass means provides a path for the substantially non-selective passageof fluid through or around the filter layer upon attainment of apredetermined pressure differential across said filter layer.

[0012] In most (if not all) embodiments, the downstream zone of theinventive filtration device is also occupied by a plurality ofadditional filter layer, each filter layer being essentially of the samebasic construction and functionality as the aforementioned upper andrearmost filter layers. These additional filter layers—each of which candesirably be provided with its own bypass means—are generally configuredand arranged to provide a serial flow path.

[0013] In operation, when a bypass seal is breached, or apressure-activated gate ruptures, or is blown out, or is otherwiseopened, a portion of the fluid will bypass the offending “clogged”filter layer, and flow sequentially downstream to the other stillless-clogged filtration layers. Parallel flow is established between the“clogged” filter layer and the filter layer immediately downstreamtherefrom. By bypassing clogged filter layers, preferably in cascadingsequence, the operative lifetime of each filter layer in the series ismore efficiently consumed.

[0014] A pressure-activated gate can either be designed for one-time useor can be made to toggle between open and closed positions as a functionof rising and falling pressure differentials.

[0015] Further, the filtration device can either employ one of thepressure-activated gates (i.e., in the upper filter component) orseveral interspersed within a single filter component and/or amongseveral filter components.

[0016] In light of the above, it is a principal object of the presentinvention to provide a filtration device having pressure-activated meansfor bypassing one or many filter layers used therein.

[0017] It is another object of the present invention to provide afiltration device comprising several filter layers, wherein apressure-breachable seal is formed at least between an upper filterlayer and the device housing.

[0018] It is another object of the present invention to provide afiltration device comprising several filter layers, wherein apressure-activated gate is disposed in at least an upper filter layerthereof.

[0019] It is another object of the present invention to provide afiltration device having pressure-activated bypasses in all of severalfilter layers used therein, except in the rearmost filter layer thereof.

[0020] It is another object of the present invention to provide theaforementioned filtration device incorporating a one-time usepressure-activated gate.

[0021] It is another object of the present invention to provide afiltration device incorporating a pressure-activated gate capable oftoggling between “open” and “closed” positions as a function of pressurewithin said device.

[0022] It is another object of the present invention to provide afiltration device wherein a pressure-activated gate is providedintegrally in at least one of the several filter layers used therein,said pressure-activated gate being essentially a discretestructurally-weakened region of said one filter layer.

[0023] With these and other objects in view, which will readily appearas the nature of the invention is better understood, the inventionsubsists in its novel combination of parts hereinafter more fullydescribed and claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In general, each of FIGS. 1 to 4 b provide schematicrepresentational illustrations, of the invention and its components. Therelative location, shapes, and/or sizes of objects are exaggeratedand/or simplified to facilitate discussion and presentation herein.

[0025]FIG. 1a illustrates a filtration device 10 according to oneembodiment of the present invention, the filtration device havingseveral filter layers (30, 33, 35, 37 and 39), the foremost of whichincludes a pressure-activated gate 20.

[0026]FIG. 1b illustrates a filtration device 10 according to anotherembodiment of the present invention, the filtration device havingseveral filter layers (30, 33, 35, 37, and 39), the foremost of whichforms a pressure-breachable seal 22 with housing 80.

[0027]FIGS. 2a, 2 b, and 2 c illustrate exemplary embodiments 220 a, 220b, and 220 c of a monolithic single-use pressure-activated gate (i.e.,constructed of a single material).

[0028]FIG. 3a and 3 b illustrate exemplary embodiments 320 a and 320 bof an assembled single-use pressure-activated gate (ie., constructed oftwo or more materials).

[0029]FIG. 4 illustrates an embodiment 420 of a pressure-activated gatecapable of toggling between “open” and “closed” positions.

[0030]FIG. 4a provides a top view of the pressure-activated gate 420illustrated in FIG. 4.

[0031]FIG. 4b provides a bottom view of the pressure-activated gate 420illustrated in FIG. 4.

DETAILED DESCRIPTION

[0032] The present invention provides a filtration device withpressure-activated means for bypassing at least one of several layers offiltration material(s) stacked or otherwise disposed serially therein.The filter device provides good and effective filtration performanceover an extended time period, relative to prior developed peers, in partdue to the incorporation of said pressure-activated bypass means.

[0033] The filtration device according to the present invention willtypically incorporate several individual filter layers arranged toprovide a serial filtration path, the most desirable arrangementcomprising an upper and a rearmost filter layer, with a number ofadditional filter layers therebetween. In another arrangement, only anupper and a rearmost filter layer are employed. Each filterlayer—regardless of overall arrangement—can comprise a single or severalstrata or beds of filtration material.

[0034] The upper filter layer is typically the “foremost” filter layer.Certain embodiments, however, may include filter layers (or otherstructure with a fluid filtering functionality) positioned above theupper filter layer. For example, it may be desirable to construct afiltration device wherein a foremost filter component is a pre-filter oflow retentiveness, and hence, not inordinately susceptible to cloggingand cake formation (cf., relatively “open” screens, sieves, and thelike). Such foremost prefilter component could literally becharacterized as an upper filter layer. However, that is not the intentof the present invention: Bypass means would provide little, if anyadvantage, in such foremost prefilter component. Rather, in defining thepresent invention, the upper filter layer is considered “upper” simplybecause it precedes the rearmost filter layer. An “upper” filter layeris not necessarily the “foremost” filter layer.

[0035] The filter layers are positioned within an external housing, suchthat when fluid is introduced into said housing—i.e., by means of afluid inlet—fluid will pass through each of said layers in serialfashion. When the flow path is serial, fluid (which typically containssubstantial solid content) passing through the stack will place agreater load on those filter components closest to the fluid inlet. Inother words, the upper filter component in the stack will become spent(cf., “clogged”, “plugged”, etc.) more quickly than the filter layersbelow. When this occurs, the filtration efficiency of the deviceplummets, compelling costly replacement or cleaning.

[0036] To mitigate the effects of clogging, without departing from goodfiltration performance, a pressure-activated bypass means is integratedinto the upper filter layer. The pressure-activated bypassmean—desirably simple and inexpensive in its construction—is configuredto allow substantially non-selectively passage of fluid through anddownstream of the upper filter layer upon the attainment within saidhousing of a predetermined pressure in the zone immediately upstream ofsaid filter layer. Thus, when the filter layer becomes excessivelyclogged, the pressure upstream mounts, and the bypass opens.

[0037] Essentially, by integrating a pressure-activated bypass into thelayer, the flow of fluid through the filtration device can bypass itwhen it becomes so clogged and/or plugged with retained solids that itcan no longer effectively pass fluid at a predetermined desired rate.Without said bypass, at this point, the entire filtration process mayhave to be stopped, so that the filter layer can either be replaced orcleaned or revitalized. The pressure-activated bypass thus provides ameans for extending the duration of a filtration process before suchmaintenance is needed. Since the physical principals underlying thepressure-activated bypass are well-suited to designs of low materialcost and/or can be integrated inexpensively within existing populardevice designs, the present invention provides an inexpensive means forimproving the duration and conduct of filtration by stacked filtercomponent devices.

[0038] The basic components of an embodiment of the filtration device isschematically illustrated in FIGS. 1a and 1 b. As shown therein,filtration device 10 comprises a housing 80 having a fluid inlet 60 anda fluid outlet 70. Housing 80 is divided into an upstream zone 40 and adownstream zone 50 by the upper filtration layer 30.

[0039] Fluid inlet 60 is designed so that it is capable of introducingfluid into the upstream zone 40 within said housing 80. Fluid outlet 70is designed so that it is capable of releasing fluid from the downstreamzone 50 within said housing 80. In most, if not all embodiments of thepresent invention, the downstream zone 50 is occupied by additionalfilter layers, e.g., layers 33, 35, 37, and 39, each of which aresubstantially similar in construction (if not identical) to the others.

[0040] Each of the filter layers 30, 33, 35, 37, and 39 can be made ofthe same or different filtration materials, for example, polypropylene,polyester, glass, polyvinylchloride, polycarbonate,polytetrafluoroethylene, polyvinylidene fluoride, cellulose, asbestos,nylon, polyethersulfone, and other polymeric (or non-polymeric)materials. Each can comprise a single stratum of filtration material orbe formed of a composite of strata of similar or different filtrationmaterials.

[0041] In respect of porosity, there is no particular requirementimposed by the present invention on filter layers 30, 33, 35, 37, and39. However, in practice, serial flow filtration devices, whereinpremature “cake formation” is a consideration, typically are those usedfor high volume primary or secondary clarifications. Such devices oftenemploy serial-arranged filter layers of a nominal pore size ranging fromapproximately 5 microns to approximately 0.1 micron. In accord with thepresent invention, the layers 30, 33, 35, 37, and 39 can all havesimilar porosities, or they can be different.

[0042] Filter layers 30, 33, 35, 37, and 39 can be stacked withinhousing 80 with no space between adjacent layer (not shown), or with aslight air space between them (see FIG. 1a), or with a spacer betweenthem (see FIG. 1b), the former and the latter being the more commonarrangements. A spacer, such as a screen, is used typically to impartrigidity and support to a stack of thin, comparatively fragile filterlayers. Such screen generally allows fluid to flow freely through itboth laterally and orthogonally. Where the filter layers are thicker(cf., filter pads), one may be able to stack them next to each other,without the need for screens or like spacers.

[0043] While filter layers 30, 33, 35, 37, and 39 have several featuresand functionalities in common, in respect of the present invention,bypass means are intentionally omitted from rearmost filter layer 39.Rearmost filter layer 39 in a typical arrangement serves as a trailingnon-“bypassable” filter layer punctuating a sequence of similar, butotherwise “bypassable” filter layers. When it becomes “clogged”, thefiltration device becomes essentially “spent”. Other filter layerspreceding the rearmost filter layer may also not be provided with bypassmeans. Regardless, the rearmost filter layer 39 certainly lacks it.

[0044] Whether provided in the foremost filter layer and/or theadditional filter layers, the bypass means has two principalembodiments, illustrated respectively in FIGS. 1a and 1 b.

[0045] In the first embodiment—shown in FIG. 1b, the bypass meanscomprises a pressure-breachable seal 22. Upon attainment of apredetermined pressure differential across the filter layer 30, the sealis breached and fluid allowed to non-selectively “leak” through seal,i.e., around the edges of filter layer 30. In a base configuration, thepressure breachable seal 22 can be provided by fitting the filter layer30 into the housing such that it abuts the inner walls of housing 80 atsome predetermined force. Those skilled in the art will be able toestablish the breaching point of a seal thusly formed by considerationand manipulation of the filter layer's morphology, porosity, rigidity,thickness, dimensions, and like physical properties. In accord with thepresent invention, the breaching point of the seal at the foremostfilter layer 30, by design, will be markedly less than that of therearmost filter layer 39, which—as indicated—is not intended to bebreached in normal operation.

[0046] As an alternative to a friction-based seal, one can also employadhesives (and like materials) to form pressure-breachable seal 22 atthe juncture between filter layer 30 and housing 80. In such case, theseal is breached as a result of either the adhesive or cohesive failureof the seal upon attainment of said predetermined pressure differential.The adhesive and cohesive properties of various polymeric materials thatcan be employed for this purpose are well reported in the scientific andpatent literature. Those skilled in the art can refer to such literatureif this embodiment is of interest.

[0047] In a second embodiment, the bypass means comprises apressure-activated gate.

[0048] Several and various configurations are available for thepressure-activated gate 20. For example, the pressure-activated gate canbe a zone of weakened pressure-resistance made of essentially the samematerial as the layer of selectively-permeable filtration material, suchas a thin section that ruptures, or a perforated area that ruptures.Alternatively, the pressure activated gate can comprise a plug insertedthrough said layer that upon attainment of said pressure differentialrange reveals an aperture. Such gate can be a molding having a notchedarea, or tabs or flaps, that are breakable or are flung open at acertain predetermined pressure differential. Even more simply, the plugcan—like a cork—be a monolithic geometric solid friction-fitted into ahole provide through said filter medium, only to be “blown out” (i.e.,blown through) at said certain predetermined pressure differential.

[0049] As a still further alternative, the pressure-activated gate canbe configured such that it is capable of opening upon reaching saidpressure differential range and then closing when below saidpredetermined pressure differential range. Such configuration willlikely involve a combination of fixed and moving parts, and may be morecostly than other embodiments. However, it provides an additionaladvantage in that it is capable of toggling between an “open” and“closed” state. This may be important for certain filtrationapplications where pressure fluctuates wildly. Under such conditions, asteep pressure spike—though intense—may be so short-lived that one maynot want a gate to remain open once pressure conditions revert back toacceptable levels.

[0050] When not made of the same materials as the filter medium, severalother materials may be considered for the manufacture of the pressureactivated gate. The following table—offered only as a guide—can beconsulted in the selection of said materials. Mold Tensile ElongationShrinkage Strength (%) Material (General) Material (Specific) (in/in)mils (psi) (Rupture) Polyethylene High Density (LNP-FF 1004) 3.5   8000Ult. 2-3 Low Density (Amoco 29081) —   4000 Yld. 300 Polypropylene Amoco1012 15-20   5400 Yld.  40 Profax 6523 19   5075 Yld. — Teflon PVDF 3  6300 Yld. — PCTFE —   5725 Ult. 150 Polysulfone PES Unfilled 7 12,200Yld. — Udel P1700 7 10,200 Yld.  50-100 Polyvinylchloride Geon 8272374-6   6000 YLd. — Geon 87242 4-6   7650 Yld. — Cellulose CelluloseAcetate 036-MH 5-8   3800 Yld. — Cellulose Butyrate 205-M 3-6   3800Yld. —

[0051] While the above table provides technical information for certainmaterials, it will be appreciated that the present invention is notlimited to the selection only of these materials. Others can beemployed.

[0052] The pressure-activated gate—of the types illustrated in FIGS. 2to 4—is incorporated into a hole provide through a filter layerapproximately midway between it center and it periphery. The areaoccupied by the pressure-activated gate is in the order of less than a ½inch diameter. This “footprint” is quite small in comparison to thetotal filter area of the device, and should not effect appreciablyfiltration performance in most circumstances. The pressure-activatedgate is typically configured to open at a pressure differential greaterthan approximately 15 to 20 psi.

[0053] Specific designs for the pressure-activated gate 20 areillustrated schematically in FIGS. 2 to 4.

[0054] In FIG. 2, the pressure-activated gate 220 a is designed as aplug fitted or otherwise inserted within a pre-made hole 32 through thefiltration medium of filter layer 30. The pressure-activated gate 220 acomprises a hole-covering seal 280 a which—as it name implies—hasdimensions sufficient to seal said hole 32, and thus prevent thedownstream passage of fluid therethrough. As shown, the generallymonolithic, uniformly thick appearance of hole-covering seal 280 isinterrupted by the provision therein of a substantially thinner region,i.e., notch 222.

[0055] Notch 222 segments the hole covering seal 280 into a number oftabs 228. These tabs 228 remain in a fixed position covering said hole32, until said notch 222 gives way to the forces of accumulatingpressure in the zone 40 upstream of said layer of filtration material30. By varying the materials, shape, dimensions of both the notch 222and tabs 228 defined thereby it is possible to configure thehole-covering seal 280 a such that it will fail at a relativelyspecific, predetermined critical pressure range.

[0056] Although the notch 222 in FIG. 2a (and FIGS. 2b and 2 c) is shownas v-shaped, there is no particular limitation in the practice of thepresent invention to its shape. The notch 222 can also be u-shaped orrectilinear in cross-section, and can form single or plural grooves,that intersect or not, on the upper and/or bottom surface of saidhole-covering seal 280 a. It can be formed during the casting or moldingof the pressure-activated gated 280 a, or it can be formed subsequently,for example, by known thermoplastic stamping or cutting technologies.

[0057] Other elements of pressure-activated gate 220 a shown in FIG. 2aare flange 224 and side wall 226. Both flange 224 and side wall 226function as means for positioning the pressure-activated gate 220 a sothat it is nested snugly within hole 32. Flange 224 provides verticalpositioning, preventing the pressure-activated gate 220 a from fallingthrough hole 32. Side wall 226 provide lateral positioning, preventingthe pressure-activated gates from shimmying from side-to-side acrosssaid hole 32.

[0058] It will be noticed in FIG. 2 that the length of side wall 226 ofpressure-activated gate 220 a is not co-extensive with the thickness ofthe filter material 30. While this is typical of a plug inserted into afilter material 30, it is not particularly significant to the practiceof the invention. Several embodiments are envisaged wherein the sidewall extends equally with or passes beyond the thickness of the filtermaterial. For example, in one embodiment, the pressure-activated gate220 a is provided with both a front flange (such as flange 224) and aback flange (not shown), such that the gate essentially clamps onto andexerts pressure on the periphery of said hole 32, thus creating atighter peripheral seal between the filter material and the gate. Insuch embodiment, the side wall—which can be of unitary design or matedelements—would be essentially co-extensive with the thickness.

[0059]FIG. 2b illustrates another embodiment 220 b of thepressure-activated gate. In contrast with the embodiment 220 aillustrated in FIG. 2A, pressure-activated gate 220 b is differentiatedby its thinner hole-covering seal 280 b—and thus, thinner tabs 228 b.One purpose for using a comparatively thinner seal 280 b is to reducethe amount of pressure required to breach the seal 280 b, i.e., byreducing the force required to flex the seal 280 b sufficiently to breakit at the notch area and subsequently bend the tabs 228 b downwards.

[0060] It will be noted from FIG. 2a and FIG. 2b, that the uppersurfaces of the pressure-activated gates are essentially flat, i.e., theupper surface of flange 224 is at the same level of the upper surface ofthe hole-covering seal 280. The feature provides advantage from amanufacturing standpoint, in that the design is quite well-suited forsingle-die injection molding methodologies. Because of the relativelylow costs associated by using only a single-die, the gate can bemanufactured at relatively high volumes, leading ultimately to lowerproduct costs.

[0061] In contrast to the embodiments shown in FIG. 2a and FIG. 2b, theupper surface of the pressure-activated gate 220 c illustrated in FIG.2c is not essentially flat. Rather, although similar in thickness to theone shown in FIG. 2b, the hole-covering seal 280 c is lower in respectof the top surface of flange 224. This format, if made by injectionmolding technologies, would likely call for the use of upper and lowermolding dies—i.e., one to mold the top relief and the other mold thebottom relief. This embodiment may be slightly more expensive tomanufacture than the gate 220 b of FIG. 2b.

[0062] The function of the inset in FIG. 2c, however, would be in thiscase to decrease the amount of pressure required to rupture the seal—apotential advantage for certain applications. This decrease can beattributed, in part, to the creation of localized force-concentratingflex points 37 at the corners of the inset that may make it easier tobend the tabs downwards.

[0063]FIGS. 3a and 3 b illustrate embodiments of the present invention,wherein the pressure-activated gate is not a one-piece unitarymonolithic part.

[0064] In FIG. 3a, the pressure-activated gate 320 a is embodied as acylindrical plug (comprising sidewall 326 a and flange 324) over-moldedor otherwise disposed around hole-covering seal 380. In this embodiment,the composition of the hole-covering seal is not limited in any broadsense. However, a desirable selection would be to use a membrane (orother like filter media), for example, a thin paper or cellulosemembrane. By employing a filter-type media as a hole-covering seal, oneis provided with another functional dimension to the plug aside frombypass functionality. In particular, by this embodiment, thepressure-activated gate provides also a filter functionality. Thus, thearea of the host filter material 30 dedicated to the gate 380—which mayotherwise constitute dead space in respect of surface filter area—canstill provides filter functionality, such that the employment of thegate will not reduce too much the primary filtering capacity of the hostfilter material 30. In light of this, it would be desirable, though notrequired, that membrane for the hole-covering seal 380 be selected suchthat it substantially replicates the filtration properties of the hostfilter material 30.

[0065] Since most membranes are not designed particularly to rupture,the point at which the seal 380 will be breached will depend on theparticular physical properties of the membrane or film selected. It willbe appreciated that in respect of commercially available membrane, thedesigns thereof are generally not struck with primary focus on rupturepoints, filtration and retention properties being more on the forefrontof manufacturers' minds. Hence, although, the physical properties beingtypically known, practitioner will realize that in making suchembodiment, they would either have to custom tailor a membrane to breakat a precise pressure range, or find external means to fine tune itsrupture. The embodiment of FIG. 3b illustrates one means by which thelatter can be accomplished.

[0066] In FIG. 3b, it will be notice that the sidewall of 326 b of thecylindrical plug is different from the sidewall 326 a of the plug inFIG. 3a. In particular, the pressure-activated gate 320 b is constructedsuch that the membrane seal 380 is held in place by a side wall 326 b,whose downstream diameter D₂ is smaller than its upstream diameter D₁.This is accomplished by the provision of a sharp annular ledge 22. Bysuch construction, when pressure is exerted upon membrane seal 380,force is exerted on the edge of the annular ledge, which—at thatpoint—is essentially acting like a knife edge. By such construction, onethus need not be bound entirely on the preset physical properties of themembrane to determine when it ruptures. This, of course, is just oneexample. Those skilled in the art can attempt other means within thescope of the invention.

[0067] The embodiments of the pressure-activated gate illustrated inFIG. 2 to 3 are all structured to provide essentially single-use gates.However, as discussed above, certain applications may find better suiteda pressure-activated gate capable of toggling between an open and closedstate. Several means of accomplishing this are available. Regardless, ingeneral, it is envisioned that a switchable gate will comprise a pluginserted through said layer of filtration material, the plug having anaperture that is capable of opening upon the attainment of the desiredpressure range, and closing when and if pressure falls beneath saidpressure range.

[0068] One specific embodiment of such switchable gate is illustrated incross-section in FIG. 4. This pressure-activated gate 420 is—like theother embodiments—a cylindrical plug inserted into a hole providedthrough layer of filter material 30. The principal components of thepressure-switchable gate 420 are its plunger and its aperture 432. Theplunger comprises an elastic gas-filled bladder 452, stem 454, andstopper 450. The plunger is held in position laterally by crossbeam 434and such that—when the pressure within the filtration device in belowthe predetermined critical range—the stopper sits snugly within aperture432, thus impeding the flow of fluid downstream therethrough.

[0069] As shown in FIG. 4a, the crossbeam 434 does not cover the lumen65 of gate 420 entirely. Fluid can pass freely into the lumen 65 (asindicated by the arrows in FIG. 2). As shown in FIG. 4b, the bottomsurface 326 of the plug, however, is a solid surface, but for theaperture. Thus, when the aperture 432 is “corked” by stopper 450, fluidwill not pass downstream.

[0070] The elasticity of the bladder 452 and the compressibility of thegaseous contents thereof are selected such that when the pressure withinthe lumen 65 rises to the predetermined critical range, the bladder 452compresses, for example, by flattening out. When bladder 452 compressesthe stopper 450 is elevated off the aperture 432, thus allowing fluid topass downstream therethrough. When the pressure is reduced, the bladder452 reverts back to its original size, pushing the stopper back into theaperture 432, preventing again the flow of fluid therethrough.

[0071] While only a few illustrative embodiments of the presentinvention have been discussed, it is understood that variousmodifications will be apparent to one skilled in the art in view of thetotality of the description herein. All such modifications are withinthe spirit and scope of the invention as encompassed by the followingclaims.

1. A filtration device comprising a housing, a fluid inlet, a fluidoutlet, and at least an upper and a rearmost filter layer, the filterlayers being made of the same or different selectively-permeablefiltration material; the upper filter layer dividing the interior of thehousing into an upstream zone and a downstream zone, said fluid inletcapable of introducing fluid into said upstream zone, said fluid outletcapable of releasing fluid from said downstream zone out of saidhousing; the upper filter layer being configured such that, uponattainment of a predetermined pressure differential across said upperfilter layer, said upper filter layer will allow substantiallynon-selective passage of fluid from said upstream zone into saiddownstream zone; the rearmost filter layer being positioned in saiddownstream zone and configured such that, upon attainment of the sameaforementioned pressure differential across said rearmost filter layer,said rearmost filter layer will not allow non-selective passage of fluidtherethrough or beyond.
 2. The filtration device of claim 1, whereinsaid upper filter layer includes a pressure-activated gate integratedinto said selectively-permeable filtration material, thepressure-activated gate capable of allowing said substantiallynon-selective passage of fluid upon attainment of said predeterminedpressure differential.
 3. The filtration device of claim 1, wherein saidupper filter layer abuts said housing to form a seal therewith, saidseal being breachable upon attainment of said predetermined pressuredifferential, thereby allowing said substantially non-selective passageof fluid.
 4. The filtration device of claim 1, wherein said upper filterlayer is held by a substantially rigid framework, said upper filterlayer and said substantially rigid framework combining to form an upperfilter component.
 5. The filtration device of claim 1, furthercomprising an additional filter layer or layers between said upper andrearmost filter layers, each said additional layer or layers being madeof the same or different selectively-permeable filtration material. 6.The filtration device of claim 5, wherein said housing is substantiallycylindrical in shape, and wherein each of said filter layers issubstantially disc-shaped.
 7. The filtration device of claim 6, whereineach adjacent filter layer is separated proximately by an air space. 8.The filtration device of claim 6, wherein each adjacent filter layer isseparated by a screen capable of allowing substantially non-selectivepassage of fluid therethrough and laterally.
 9. The filtration device ofclaim 1, wherein said pressure-activated gate is a discrete zone ofweakened pressure resistance made of essentially the same material ormaterials as said filtration medium.
 10. The filtration device of claim1, wherein said pressure-activated gate comprises a plug insertedsubstantially through the filter medium of said first filter component,said plug having an aperture that is opened upon said attainment of saidpredetermined pressure differential.
 11. The filtration device of claim10, wherein said aperture is capable of closing when pressure fallsbeneath said predetermined pressure differential.